Appendix A
Lightning Protection Recommendation
a.1.
Introduction
What do you do with a 2 million volt pulse
pushing 220,000 amps of current into your
transmitting plant? Like the 500 pound go-
rilla you let it do what ever it wants to. There
is not much that can be done to protect
against a major direct lightning strike. This
is called a significant impulse lightning
stroke. It usually lasts less than 100 micro-
seconds and is most destructive to electronic
equipment because it contains huge
amounts of high frequency energy.
Here are some examples of this damage:
•
Melted ball and horn gaps.
•
Ground straps burned loose.
•
H.V. rectifier stacks shorted.
•
Massive arc marks in the output circuit
of AM transmitters.
•
Ball lightning traveling into building on
outer conductor of transmission line.
Figure A-1 is a map of the United States
that shows the number of lightning days you
can expect in any year. You fellas in Colo-
rado, New Mexico, and Florida need light-
ning rods on your hats.
Figure A-2 shows the incidents to tall
structures. A triggered event is one that hap-
pens because the tower was present. With-
out the tower the strike would not have
occurred.
a.2.
Enviornmental Hazards
There are devices and procedures that do
offer protection from lessor environmental
hazards than lightning. Some of these
anomalies are listed and defined:
a. Over voltage/under voltage (brownout).
Where the lines voltage differs from the
nominal RMS for longer than one cycle.
•
Remedy - Automatic voltage regulators,
preferably individual regulators on each
phase. This can only be accomplished
when the power feed line is delta or
4/wire wye connected. (See Figure A-3.)
b. Single phasing. This is where one leg of
the three phase service is open.
•
Remedy - Protection afforded by a loss
of phase detector. Without protection
power transformers and 3 phase motors
over heat.
c. Radio frequency interference (RFI). This
is something we must design into all of our
transmitters, however, you may purchase
equipment that is susceptible, is not pro-
tected, and develop problems.
•
Remedy - RFI filters on the ac lines and
control lines are sometimes effective.
Sometimes the entire device must be
enclosed in an RF free space.
d. Electromagnetic pulse (EMP). This is a
interfering signal pulse that enters the
system by magnetic coupling (trans-
former). Generally caused by lightning.
•
Lightning from cloud to cloud produces
horizontally polarized waves while
lightning from cloud to earth produce
vertically polarized waves. The waves
couple into the power lines and transmis-
sion lines causing large induced voltage
that destroy high voltage rectifier stacks
and output circuit faults. High frequency
energy is coupled back into the transmit-
ter causing VSWR overloads. (See Fig-
ures A-4 & A-5.)
•
Remedy - Ball or horn gaps at the base
of the antenna prevent the voltage from
exceeding some high potential. Tran-
sient suppressor devices on the input
power lines remove excessive voltage
spikes. Buried power and transmission
lines will reduce the amount of coupled
energy to a great extent. This does not
totally eliminate the problem because
there are currents traveling in the earth
when lightning strikes close to the sta-
tion which prefer to travel on the metal
conductors.
e. Surge. A rapid increase in voltage on the
power lines usually caused by lightning.
The duration is less than 1/2 cycle and
can be very destructive.
•
Remedy - Transient protectors are very
effective in preventing damage to the
equipment when properly designed and
installed. (See Figure A-7.)
Significant Lightning Stroke Characteristics
Charge Range
2 to 200 coulombs
Peak Currents
2,000 to 400,000
Amperes
Rise Time to 90%
300 Nanoseconds to
10 Microseconds
Duration to 50%
100 Microseconds
to 10 Milliseconds
Potential Energy at
99%
1010 Joules*
* Only a small portion is manifested in a
surge, usually less than 10,000 Joules.
a.3.
What Can Be Done?
Installation of the transmitter building, an-
tenna tuning unit if applicable, and antenna
should be done so that the risk of destruction
due to lightning is minimal and the effi-
ciency of the over all system is maximized.
To do this, separate ground systems should
be installed for the building and antenna.
This forces all of the RF return currents to
flow in the transmission line shield. The
coax can be buried below the antenna
ground plane to still further reduce the RF
current coupled to it.
Figure a-1. Isokeraunic Map of the United States
Showing Lightning Days Per Year
a-1
WARNING: Disconnect primary power prior to servicing.
